1. Field of the Invention
The present invention relates to an ink ribbon comprising an infrared ray-emitting fluorescent substance which is excited by infrared ray and emits light in the infrared region. In particular, the present invention relates to an ink ribbon having an ink layer which contains a fine particle form infrared ray-emitting fluorescent substance having excellent light-emitting characteristics.
2. Description of the Related Art
To prevent forgery and keep secrecy, an infrared ray-emitting layer which is not be visible to the naked eye, such as a stealth bar code, is heat transferred on a printed material such as a brochure or catalogue using an ink ribbon comprising an infrared ray-emitting fluorescent substance which emits light in the infrared region (see Japanese Patent KOKAI Publication Nos. 9607/1978 and 22326/1979) and the code information is read by an apparatus for optically reading to obtain information of goods such as properties and a price or varied personal information (see Japanese Patent KOKAI Publication No. 9600/1978). It has been tried to heat transfer an infrared ray-emitting layer on a plastic substrate such as a prepaid card, an ID card and a magnetic card, or a magnetic layer using such an ink ribbon. However, an infrared ray-emitting fluorescent substance which is conventionally used in the ink ribbon has a large particle size. For example, a phosphorus base infrared ray-emitting fluorescent substance of the formula: EQU QD.sub.1-x-y Nd.sub.x Yb.sub.y P.sub.4 O.sub.12
wherein Q is at least one element selected from the group consisting of Li, Na, K, Rb and Cs; D is at least one element selected from the group consisting of Sc, Y, La, Ce, Gd, Lu, Ga and In; x is a number of 0.05 to 0.999 and y is a number of 0.001 to 0.95, provided that a sum of x and y (x+y) is 1.0 or less is disclosed in Japanese Patent Publication No. 40594/1978) and often used but its particle size is 7 .mu.m or larger.
Because of such large particle size, it should be ground to a particle size of 0.1 to 2 .mu.m when it is used in the ink ribbon. But, by grinding, crystallinity or composition of the infrared ray-emitting fluorescent substance is deteriorated and the particle shape becomes a needle shape so that the particle size distribution is broadened and dispersibility is worsened, whereby the dispersing apparatus is damaged. In addition, the intensity of the emitted light is greatly decreased and duration of afterglow is shortened. Accordingly, such infrared ray-emitting fluorescent substance may not be practically used when a thickness of the ink ribbon is 1 to 10 .mu.m.